The present invention relates to a photorefractive apparatus, i.e. an apparatus where a photorefractive effect is liable to occur.
Explanations of the photorefractive effect are provided in:
(1) Photorefractive effect, David Pepper, Jack Feinberg, Nicolai Kukhtarev, For the Science, No. 158, December 1990, pages 58 to 64.
Reference should also be made to:
(2) Topics in Applied Physics, Springer Verlag, Vol. 61, Photorefractive materials and their applications, Volume 1, Chapter 8, The Photorefractive Effect in Semiconductors, Alastair M. Glass and Jefferson Strait.
The apparatus according to the invention can be used as a rewritable holographic recording apparatus.
As a function of the components of the apparatus according to the invention, said rewritable holographic recording apparatus can be sensitive in the visible range or in the near infrared range.
The present invention has, inter alia, as applications all those of a rewritable holographic recording apparatus and in particular the deflection of light beams, the formation of reconfigurable optical interconnections, the production of phase conjugation mirrors and the provision of signal optical treatment or processing devices, which can be of an analog or digital nature.
In the case of analog signals, the present invention e.g. applies to the production of optical correlation devices and in the case of digital signals to the production of arrays of optical logic gates.
For some years now, photorefractive materials have constituted the most widely investigated class of rewritable holographic recording apparatuses for all dynamic holography applications.
The principle of the photorefractive effect is as follows. The charge carriers are photoexcited under the effect of a non-homogeneous illumination such as an interference pattern and are non-homogeneously redistributed, thus creating a space charge field. The latter induces by the electrooptical effect a variation of the refractive index, which is the "image", apart from a possible spatial displacement, of the initial interference pattern. The two fundamental elements of the photorefractive effect are consequently photoconductivity and the electrooptical effect.
Most frequently, the electrooptical effect is the Pockels effect and it is then necessary to use photorefractive materials in the form of solid monocrystals, which can be in the form of a parallelepiped, whereof each of the edges is a few millimeters.
Compared with several other non-linear optical effects, the photorefractive effect has an original characteristic. Thus, it is sensitive to light energy per surface unit and not to the light intensity of the write radiation.
This is due to the integration effect inherent in the formation of the space charge field, the number of displaced charges per time unit being proportional to the photon flux.
Thus, the variation of the refractive index dN increases, in a time called the response time and which is designated "tau", which is inversely proportional to the incident light intensity until it reaches a limit value dNmax.
Known photorefractive materials can be classified in two categories from the performance standpoint:
slow materials having a significant effect on saturation, such as LiNbO.sub.3, BaTiO.sub.3, KNbO.sub.3, (BaSr)Nb.sub.2 O.sub.6 (SBN), PA1 fast materials having a limited effect on saturation, such as sillenites (Bi.sub.12 SiO.sub.20, Bi.sub.12 GeO.sub.20, Bi.sub.12 TiO.sub.20) and semiconductors (GaAs, InP, CdTe, GAP).
When account is taken of the refractive index variation per light intensity unit dN/I, which is a function of the figure of merit N.sup.3 .multidot.r/eps (in which I,N,r and eps respectively represent the intensity of the write radiation, the refractive index of the material, the effective linear electrooptical coefficient of the material and the dielectric constant of said material), it can be seen that all known solid photorefractive materials have identical characteristics to within a factor of 10.
The interest of the materials GaAs, InP and CdTe is of having a significant sensitivity to the wavelengths used in the field of optical telecommunications.
However, the known photorefractive materials suffer from a disadvantage. It is not possible to find materials which simultaneously have a short response time to optical excitation and a high refractive index variation dN.
Various solutions have been tried for solving this problem.
1. Application of an electric field for increasing the space charge field.
This has been successful with sillenites and semiconductors. However, it leads to a certain increase (10 to 100) of the time constant compared with operation in a zero field and particularly leads to the appearance of a photocurrent, as a result of the application of an electric field, limits the maximum intensity which can be withstood by the photorefractive crystal and therefore the response time cannot be made as short as when no field is applied.
2. Application of an electric field for increasing the effective electrooptical coefficient.
If the use wavelength is close to that corresponding to the width of the forbidden band of the material, the electrorefraction (Franz-Keldysh effect) becomes high and can lead to an effective electrooptical coefficient (proportional to the field) which is several times higher than the linear electrooptical coefficient.
This resonant effect (unlike the standard photorefractive effect), combined with the space charge field increase effect, makes it possible to achieve the highest limit values dNmax in solid semiconductors.
However, the "speed" limitations due to the photocurrent remain the same as in the case referred to in 1, i.e. the response time to an optical excitation remains relatively high.
3. The use of a multiple quantum well structure.
It is known that the electrorefraction effect can be significantly increased on passing from a solid material to a multiple quantum well structure. This has applications in the field of light modulation in guided configuration or perpendicular to the substrate. Reference can be made to the following document in this connection:
(3) Resonant photodiffractive effect in semiinsulating multiple quantum wells, D. D. Nolte, D. H. Olson, G. E. Doran, W. H. Knox and A. M. Glass, J. Opt. Soc. Am.B, Vol. 7, No. 11, November 1990, pages 2217 to 2225.
In connection with the photorefractive effect, document (3) has demonstrated the possibility of obtaining significant diffraction efficiencies (through 1 micrometer of active material). The method proposed in (3) consists of making semiinsulating a GaAs/AlGaAs multiple quantum well structure by introducing protons.
Compared with the methods described hereinbefore, the electric field is only used for increasing the effective electrooptical coefficient and not for a contribution to the displacement of the charges in the multiple quantum well structure. Therefore, the advantages of the method described in (1) are lost, but it is possible to use short response times.
Another interest of the multiple quantum well structure compared with a solid structure is the possibility of adjusting the operating wavelength for a given application.
However, the diffraction efficiencies obtained with the method described in (3) are approximately 10.sup.-5 and are therefore too low for the applications envisaged hereinbefore in connection with rewritable holographic recording apparatuses.
In order to obviate this disadvantage, one possible solution consists of increasing the thickness of the active material but, in this case, the growth method and more particularly the implantation method are unsuitable and it is therefore necessary to seek an optimized multiple quantum well structure for these electrooptical effects and containing a deep center with an energy level and a concentration which are carefully chosen, which can make optimization difficult.
Thus, with known photorefractive materials, it is impossible to simultaneously have a high speed for establishing the refractive index system, said speed being more particularly linked with the mobility mu of the charge carriers and a high amplitude for the photoinduced index system, said amplitude being linked with the electrooptical figure of merit value N.sup.3 .multidot.r/2.
Thus, the fastest of the known materials is InP:Fe (mu approximately 3000 cm.sup.2 /V.multidot.s) and systems can be recorded by the photorefractive effect in semiconductors such as GaAs, InP and CdTe in a few dozen picoseconds with luminous fluxes of approximately 1 mJ/cm.sup.2 at 1.06 micrometer. However, the figure of merit of InP is only 25 pm/V.
Conversely, certain materials having a perovskite structure have a figure of merit of approximately 1000 pm/V but with mobilities well below 0.1 cm.sup.2 /V.multidot.s.
It is hardly likely that it will be possible to find a photorefractive material simultaneously having the necessary speed and high amplitude referred to hereinbefore.
In the field of low light intensities (approximately 10 mW/cm.sup.2), which are e.g. supplied by diode-pumped Nd:YAG lasers, it is possible to apply a continuous electric field of approximately 10 kV/cm to InP in order to partly compensate the weakness of the electrooptical coefficient (the response times are under these conditions approximately 50 to 100 ms), but the current increase in the presence of high intensities required for lowering the response time, although only 1 microsecond, makes it impossible to use this process in the case of high light intensities.